In the first two
parts of this series of articles we presented a spectral analysis of new and
used 400-watt metal halide lamps. In this article we will analyze the spectral
output for the 250-watt lamps.

The 250-watt lamps are not as popular as the 175- or 400-watt lamps because
of the limited choices, although some new lamps have recently been introduced.
The seven lamps shown in Table I were used for our new lamp analysis.
These lamps were burned in for 100 hours in order to stabilize the output.

TABLE INEW LAMPS TESTED

Brand name

Color
temperatureclaimed

Coralife

5500 Kelvin (K)

Iwasaki

6500 K

Coralife (new)1

10,000 K

Coralife

20,000 K

Double-ended HQI

10,000 K

Tungsram2

6000 K

Coralife

6500 K

1 = Coralife has recently
introduced this new 10,000 K, 250-watt lamp that is being made in
Germany.2 = The Tungsram lamp was sent to us from
Australia.

The lamps were placed 18 inches from the LI-1800 spectroradiometer and
powered by a MagneTek M58 ballast. The double-ended HQI lamp was run using the
recommended HQI ballast.

Spectral change during burn-in phase

Most lamp manufacturers recommend a
burn-in period of about 100 hours. However, many hobbyists do not burn in the
lamp in a separate fixture outside the aquarium. We were curious as to how the
spectrum changed during this burn-in period.

Figure 1 shows the spectral curve of the Iwasaki lamp at various
stages of its burn in. As you can see in the chart the lamp shifts in spectral
output when it is new. As the lamp reaches about 93 hours it has stabilized and
there is almost no change between the 93-hour curve and the 82-hour curve. The
change in “photosynthetic photon flux density” (PPFD) and “correlated color
temperature” (CCT) is shown in Table II.

New Lamp analysis

The first analysis done on the seven samples was a
direct comparison of the spectral outputs of the lamps, along with the PPFD. The
spectral plots are shown in Figure 2 and the PPFD is shown in Figure
3. As seen from the data, the Iwasaki 6500 K and the double ended HQI 10,000
K lamps have higher output than most of the other lamps over most of the 400 to
700 nanometer (nm) range. The Tungsram 6000 K lamp was a little better
spectrally than the Coralife 5500 and 6500 K lamps, but it still did not reach
the performance of the Iwasaki lamp or the double-ended HQI.

The double-ended HQI lamp is very different from the other lamps tested. It
is double ended, while all the others have a single screw-type base. It lacks
the outer ultraviolet (UV) filtering glass bulb of the other lamps. This lamp
must be used in an enclosed fixture with an UV protective shield. It does have a
very nice color and has the best spectral curve of any of the higher CCT lamps
tested.

The next comparison we did was to calculate the PPFD over the
“photosythetically available radiation” (PAR region; 400 to 700 nm) of the lamp
output. This value is a good indicator of a lamp’s ability to support
photosynthesis. Table III and Figure 3 show that the double-ended
10000 K lamp has over twice the PPFD of the Coralife 5500 K, 6500 K and 10,000 K
lamps and the Tungsram 6000 K lamp, and three times that of the 20,000 K lamp,
but is only about 20-percent higher than the Iwasaki 6500 K lamp. The
double-ended 10,000 K lamp performs very well, giving off nice blue-white light,
and still offers a very respectable PPFD.

The CCT was also computed for each lamp and is also shown in Table
III. The CCT for the 6500 K lamp was higher than the rating, whereas for the
5500 K Coralife lamp it was much lower than the lamp designation would suggest.
For the 10,000 and 20,000 K Coralife lamps the CCT could not be calculated.
These lamps appear too much like a monochromatic source for an accurate CCT
calculation. The double-ended HQI 10,000 K lamp has a measured CCT close to
12,000 K, which gives it its nice blue-white appearance.

TABLE IIIPPFD and CCT for thenew 250-watt lamps

Lamp Type

PPFD

CCT

6500 K Iwasaki

104.50

7457

5000 K Coralife

58.34

4585

10,000 K Coralife

51.57

not applicable (na)

20,000 K Coralife

37.24

na

10,000 K HQI

128.80

11,723

6000 K Tungsram

56.00

8152

6500 K Coralife

53.38

5339

Finally, the spectral output of the lamps was divided into color ranges to
compare the distribution of their energy output (see Figure 4). Aquarists
are often concerned with the quantity of radiation in the violet and blue range
— hence the propensity to use 10,000 and 20,000 lamps. P Another misconception
of most aquarists is that the higher CCT lamps have more output in the blue
range. As the data in Figure 4 show, the Iwasaki 6500 K lamp has a total
PPFD in the blue and violet range of 29.56, when compared to the 20.47 for the
10,000 K Coralife lamp, and 18.85 for the 20,000 K lamp. The only higher CCT
lamp that we tested that had more blue and violet light than the Iwasaki 6500 K
is the HQI lamp, at 52.95. However, it does have a very strong violet output.

The 6500 K lamp is also strong in the yellow and green ranges, which explains
the yellowish-green look reported by aquarists using this lamp. In the
double-ended HQI lamp this yellowish-green look is compensated for by the large
output in the violet range, giving it a very pleasing look.

The UV output of the various lamps is shown in Figure 5. Comparing the
UV output in this chart is somewhat misleading. The double-ended lamp has much
higher output thaen the others, but remember this lamp requires a UV filter,
which would bring this number down.

Figures 6 through 12 show the PPFD in six ranges as a percent
of the total PPFD. The 5500 K lamp has over 50 percent of its total output in
the yellow and orange range. The 10000 K Coralife lamp has 67 percent of its
PPFD in blue and violet range. The 6500 K and the double-ended HQI lamps both
show nearly uniform output. These figures show one interesting result in that
the 20,000 K Coralife lamp has a more evenly distributed output than the 10,000
K Coralife lamp.

The double-ended HQI lamp is a good example of the way the human eye is
fooled by color. The lamp is very strong in the violet and a yellow color, but
the light from this lamp looks very blue-white in color.

Used 250-watt lamps

Used lamps were requested from aquarists all over
the country in an attempt to analyze the change in spectral output over time. It
was a lot easier to get used lamps of ages around one year or more, rather than
used lamps in the three-, six- or nine-month range of use. Thus, we currently
lack data in the intermediate ranges of the life of the lamps.

The lamps solicited for the used lamp evaluation were the same as the ones
solicited for the new lamp evaluation. However, we were very disappointed that
we were not able to obtain used samples of the Coralife 6500 K, the double-ended
10,000 K or the Tungsram 6000 K lamps. We did get good samplings of the 6500 K
lamp by Iwasaki, the 5500 the 10,000 (old lamps) and the 20000 Ks from Coralife.
The spectral characteristics of these lamps were compared to a new lamp in each
category.

There are a few limitations that arise due to the testing strategy used:

The initial spectral distribution is not known, so it may be difficult to
ascertain the role of the variation in the initial spectral distribution of the
lamp.

Often, the exact number of hours the lamp was used was not known. The age
was given by aquarists in the total duration of time the lamp was in use.

The operating environment for the lamps may be very different, leading to
variation in the lamp output arising from the variation in operating conditions.

In spite of these limitations we felt it is of some educational value to look
at the spectral distributions of the lamps during various points in their life.
We’re hopeful some conclusions can be drawn.

The Iwasaki 6500 K lamps

Figure 13 shows the spectral plots of
three used 6500 K lamps as compared to two new ones. As you can see from the
figure, there is good similarity between the curves of the different lamps,
aside from the second one-year-old lamp. The curve of this lamp has a spectral
curve much different from what is normally seen in a 6500 K lamp, indicating
that the lamp may have been defective. As seen with the 400-watt Iwasaki lamps,
the 250-watt lamps also tend to lose more intensity in the violet/blue end of
the spectrum range, evidenced by the wider spread in the spectral distribution
for those wavelengths.

Table IV shows some interesting data. The PPFD for some of the used
lamps showed up as being slightly higher than one of the new ones tested. This
could simply be due to the variation among the initial PPFD of the lamps, or it
could be that the new lamp was not a very good lamp. This table also shows that
there is actually some variation between new lamps. One of the new ones has
about 10-percent higher PPFD than the other.

The important point here is that even after as long as two years of use the
lamps were within 20 percent of each other. One could infer from this that these
lamps could easily be used beyond the one-year period.

Another interesting observation that can be seen in Table IV is that
the CCT for the defective lamp is nearly the same as that of a lamp that has
twice the PPFD. This shows that the ability of a lamp to provide a good supply
of photosynthetic radiation can not be determined by CCT alone. The defective
lamp has a PPFD that is less than half of any of the other lamps.

The Coralife 5500 K lamps

Figure 14 shows the spectral curves for
a group of used 5500 K Coralife lamps. The spectral plots show a strong
similarity in the spectral curves with a fairly small spread. There is a sharper
drop in the peaks, with small drops over the rest of the spectrum.

Once again, as seen from the data in Table V, it is difficult to draw
any strong conclusions, other than the fact that these lamps could be used
longer than a year. The variation in the PPFD after a year of use could well be
due to the initial variation in the lamps. This may show some substantial
difference from lamp to lamp, but a large group of new lamps would be needed to
confirm it.

The Coralife 10,000 K lamps (old version)

Coralife has recently
introduced a new 10,000 K lamp. The results presented in this section are for
the old version of the lamp. The used 250-watt 10,000 K lamps that were tested
show a typical spectral curve for higher color temperature lamps. There is a
very large spike at 450 nm. Figure 15 shows the nearly monochromatic
spectral curve of these lamps.

Table VI shows the PPFD of each of the 10,000 K lamps. This set of
lamps seems to have the widest fluctuation in the output of the used lamps.
Unless the quality of this lamp can be established it is almost impossible to
make any conclusions about it, other than its inconsistent performance. Because
of this inconsistent output you really don’t know what you are buying.

The Coralife 20,000 K lamps

The data for the 20,000 K Coralife lamps
(see Figure 16) shows a plot very similar to the Coralife 10,000 K lamps.
However, these lamps seem more consistent and predictable in performance. There
is a large drop in the peak output at the 450 nm range, as a function of the age
of the lamps.

These lamps tended to lose up to 37 percent of their output within a year.
This indicates that these lamps should probably be changed earlier than the
lower CCT lamps.

Tungsram 6000 K lamps

We were only able to obtain one new and one used
Tungsram lamp. The spectral plot for the new one compared with the used one is
shown in Figure 17. The PPFD for the new lamp was 56.00 and the used one
with 2900 hours of life was 84.33. Without more lamps of this type it is
difficult to find a reason for the low PPFD of the new lamp, and the difference
in the general shape of the curves.

Further, because these were lamps from Australia it is quite likely the
ballasts we used (M58) do not match the ones specified for these lamps.

Conclusion

In this article we presented the spectral analysis of
250-watt metal halide lamps used in the hobby. The data presented in this
article are intended to provide a better understanding of the light output of
the lamps commonly used in the reef hobby. We do hope this data will help you
make more informed decisions when choosing which lamp is best for your reef.

If you subscribe to the “more PAR is better” theory, then obviously the best
choice is either the double-ended 10,000 K lamp or the 6500 K Iwasaki lamp. If
you subscribe to the “more blue is better because corals are found in water
where the higher wavelengths are filtered out” theory, then it’s worth noting
that the 6500 K Iwasaki lamp had higher output in the violet/blue range than the
10,000 and 20,000 K Coralife lamps. The double-ended 10,000 K lamp is the best
combination, offering both the best PPFD and the more blue color many reef
hobbyists are interested in.

As mentioned in pervious articles, we would like to continue collecting data
on metal halide lamps. We would like to appeal to aquarists to help by loaning
us the lamps for testing. If you have lamps that you would be willing to provide
for the study, please contact Sanjay Joshi or Dave Morgan, in care of
Aquarium Frontiers.

Acknowledgements. We would like to thank several aquarists for lending
us their lamps — Richard Harker, Robert Singer, Mike Fontana, John Newton, Dana
Riddle, Chris Paris, Dallas Warren, Brian Griffin, Coralife and Hamilton
Technologies. We would also like to thank Dr. Paul Walker of Penn State
University and LiCor, Inc., for the use of their spectroradiometers and
darkrooms for testing the lamps.